Osteology of Circus hudsonius. 5 
circular opt:c foramen, and the foramina more external to it are 
quite distinct from each other, which is by no means the rule gen- 
erally among birds. 
The outline of the olfactory foramen, leading into the brain case 
is very irregular, and the wall in its immediate neighborhood is 
thinned to the extent uf perforation in one specimen before me, 
while in another two minute formina occur, just large enough, on 
either side, to admit the passage of the nerves, and the aforesaid 
perforation is much smaller. Quite an extensive osseous flap is 
thrown out to shield the opening to the ear behind. This latter 
aperture is comparatively very large, the opening being fully equal 
in size to the corresponding one in a specimen of Falco r. gyrfalco 
from Alaska, which I find in my collection, and, as we know, a 
very much larger Hawk than Cz7cus. We have to account for 
this, in the well-known fact, however, that in the structure of the 
ear parts the Marsh Harrier approaches the Owls. In the upper 
part of the recess, formed by this aural cavity, the double head of 
the guadrate articulates, the outer head with the squamosal, the 
inner one with the bony wall within. This bone then becomes 
twisted on itself, to support below the usual articular facet 
for the mandible, which facet is quite narrow from before 
backwards, and rather long transversely. It presents two ar- 
ticular surfaces, an outer and an inner, connected by a narrow 
isthmus posteriorly, and separated by a shallow concavity an- 
teriorly. 
The quadrate throws inwards a stumpy orbital process, the 
anterior surface of which lies in the same plane with the general 
anterior surface of the bone, it being directed upwards, forwards 
and outwards. On the’posterior surface of the quadrate we find 
a longitudinal depression coming down from between the two 
heads mentioned above, which harbors one of the pneumatic 
foramen, the other being found at the base of the orbital process 
on this aspect. The peculiar form of the cranial vault with the 
bulging supra-occipital srominence, should be noted on this lateral 
aspect of the sknll. 
Upon a superior view of the skull of the C7rcus (Fig 2), the 
principatl poins to be observed are, among others the position of 
the elliptical, osseous nares; the direction of the cranio-facial 
suture, which is not in this Hawk drawn directly across in a 
transverse line, as it is in /al/co spaverius for instance. It is to 
